Contractile force in muscle fibers is generated by crossbridges that form between the myosin-containing and actin-containing filaments and hydrolyze ATP. During shortening, the two types of filaments slide relative to each other. Structural changes associated with crossbridge formation were measured by X-ray diffraction, using skinned fiber preparations from rabbits and frogs. An electronic, position-sensitive, X-ray detector was developed and interfaced with a computer. This system made it possible to record and analyze two-dimensional X-ray diffraction patterns with high quantum efficiency. The first recording from skinned rabbit psoas fibers of the tropomyosin-sensitive reflection was obtained, which opens the way to further functional study of this important regulatory protein. The myosin-containing and actin-containing filaments in skinned rabbit fibers, which are axially aligned under physiological conditions, were found to remain straight when fibers in rigor (ATP free) were caused to expand laterally by decreasing ionic strength and raising pH. Thus, non-isotropic sarcomere expansion and associated filament "bending" could be ruled out as a cause of the forces that develop under these conditions. Radiation inactivation analysis of skinned rabbit fibers gave a target size of 3x1 million daltons for both calcium-activated force development and passive resting tension. This value is close to the size of the cytoskeletal protein, titin. The same target size for active and passive tension indicates that the position of the myosin-containing filaments in the sarcomere may be stabilized axially by titin filaments which run from Z line to Z line. The overall goal of the project is to find out how the actomyosin system in muscle fibers produces force and motion, and how the mechanical responses are regulated.